The applicant of the present invention proposed a process for producing molten iron as a new iron making process instead of a conventional blast furnace process and a smelting reduction process. In this process for producing molten iron in which a rotary hearth furnace and a iron bath type melting furnace are used in combination, after carbon composite iron oxide briquettes each containing an iron oxide source and a carbonaceous reducing agent are processed by heating reduction in the rotary hearth furnace to form solid reduced iron, this solid reduced iron is fed to the iron bath type melting furnace, and a carbon material supplied as a fuel is combusted with oxygen in the iron bath type melting furnace, so that the solid reduced iron is melted to form molten iron (see Patent Documents 1 and 2).
However, through subsequent investigation carried out by the inventors of the present invention, it was found that in the iron bath type melting furnace, since a large amount of molten slag (hereinafter simply referred to as “slag” in some cases) is produced which is derived from gangue components in the iron oxide source and ashes of the carbonaceous reducing agent and the carbon material, when abnormal foaming of this large amount of slag once occurs in the furnace, it is difficult to suppress this slag foaming, and because of this slag foaming, slag and/or base metal gushes out of the furnace. As a result, yield loss and/or operation interruption may occur, an exhaust gas system may be blocked by slag and/or base metal which is carried therein, and in addition, since the heat capacitance of slag which is light-weighted by the above foaming is small, the slag is cooled and solidified when it is discharged, so that a taphole is disadvantageously clogged thereby.
In addition, as a process for melting scrap in a top-blown converter type reaction furnace by heat generated when a carbon material is combusted using an oxygen gas, a process has been disclosed in which after scrap is smelted, the total amount of produced molten iron is discharged by tilting the furnace while the total or at least 50% of slag is allowed to remain therein, and molten iron is charged into the furnace for next charge (see Patent Document 3).
However, according to this process, since the furnace must be tilted for each charge, besides the outflow of the molten iron and the carbon material caused by the tilting, heat dissipation loss is also increased, and hence the productivity of molten iron is disadvantageously decreased.
In addition, as a process for continuously producing molten iron from scrap and/or iron ore using a cylindrical furnace, the following process has been disclosed. Accordingly to this process, when a low-fluidity slag having a low basicity is formed in melting, only molten iron is discharged from a discharge port in molten iron discharge, and the low-fluidity slag remains in the furnace. Subsequently, when the amount of the slag accumulated in the furnace reaches a value limit by repeatedly performing the above molten iron discharge operation, the basicity and/or the temperature of the slag is increased to increase the fluidity thereof, and the slag is then discharged (see Patent Document 4).
According to the process for producing a molten iron described above, since tilting of the furnace is not performed, and slag separation is performed without providing an additional slag discharge port, the stability of continuous operation and the improvement of iron yield can be obtained. However, since a large amount of slag is allowed to remain in the furnace for a long period of time, when an oxygen jet flow passes through a slag layer having a large thickness, the slag is oxidized to form peroxide slag, and abnormal foaming and bumping are liable to occur. As a result, blowing may be inevitably interrupted, and stable melting operation may not be continuously performed in many cases. In particular, since having a small apparent density, reduced iron is liable to stay in a slag layer having a large thickness, and hence bumping is facilitated by air bubbles (hereinafter referred to as “CO gas bubbles” in some cases) of carbon monoxide gas generated in the melting. In addition, when the basicity and the temperature of slag are changed between the melting and the slag discharge, the control becomes very complicated. Furthermore, when a large amount of slag is present, since a carbon material is not likely to reach a molten iron surface, carburization of molten iron by the carbon material is inhibited, and as a result, production of molten iron becomes difficult.
In addition, as a process for improving discharge of molten iron and slag, a process has been disclosed in which heating is performed by induction coils which are wound around a hearth bottom of a blast furnace and that of a blast furnace type melting furnace, and the temperatures of the hearth bottom, molten iron discharge port, and slag discharge port are increased (see Patent Document 5).
However, according to this process, since an expensive induction heating apparatus is additionally required in order to increase the temperature of slag, cost of equipment is increased, and in addition, labor hours for maintenance are also disadvantageously increased.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-176170    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-257545    Patent Document 3: Japanese Unexamined Patent Application Publication No. 8-209218    Patent Document 4: Japanese Unexamined Patent Application Publication No. 5-239521    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2001-241859